Proper skeletal muscle function is dependent on activation and repression of numerous genes in satellite cells, which are tissue-specific stem cells, as well as in multinucleated contractile myofibers. Gene expression is regulated in part by controlling the localization of proteins with nuclear functions, such as transcription factors and chromatin remodeling enzymes. The subcellular localization of these nuclear proteins must be tightly controlled because altered nuclear import or export could result in aberrant muscle mass and function. How key nuclear regulatory proteins gain access to nuclei in satellite cells and myofibers is unknown. Most transport between the nucleus and the cytoplasm is mediated by nuclear pore complexes in concert with nuclear transport receptors that recognize cargo proteins and mediate transport through these nuclear pores. Our long-term goal is to understand the function of the nuclear transport machinery in normal and aged muscle and how it contributes to control of gene expression. Based on our preliminary data together with published literature, we hypothesize that perturbations in the nuclear transport machinery (nuclear import receptors and nuclear pores) alter muscle physiology. This proposal uses complementary in vitro and in vivo approaches to probe different components of the nucleocytoplasmic transport machinery in satellite cells and myofibers in order to obtain an integrated analysis of nuclear transport. Thus, we will interrogate the function of specific nuclea import receptors in satellite cells (Aim 1) and the selectivity of specific nuclear import pathways for individual nuclei of multinucleated myofibers (Aim 2). In addition, we will assess nuclear pore selectivity in skeletal muscle nuclei in response to aging and oxidative stress (Aim 3). Understanding how nucleocytoplasmic transport is regulated in skeletal muscle will lead to a greater understanding of how external signals are sensed by muscle cells and translated into changes in gene expression necessary for tissue homeostasis. These analyses may provide new targets for preventing loss of muscle mass with aging, injury, or disease. In addition, our analyses may enhance the efficiency of cell therapeutic approaches that rely on fusion and nuclear reprogramming of individual donor cells with multinucleated myofibers.